<code>validateEmbedding</code> - a function that allows you to control what elements can be embedded to
                              what other elements when the paper is put into the <code>embeddingMode</code> (see above).
                              The function signature is: <code>function(childView, parentView)</code> and should return
                              <code>true</code> if the <code>childView</code> element can be embedded into the <code>parentView</code> element.
                              By default, all elements can be embedded into all other elements (the function returns <code>true</code> no matter what).
                